An optical communication system using an optical fiber as a transmission line has advantages such as in transmission ability of mass information due to a broadband characteristic of the optical fiber and low induced noise transmission. Therefore, such an optical communication system will be used widely in various fields. In such an optical communication system, an optical switching system for optical wavelength-division and time-division multiplexed signals is used in which an input optical signal having a specific wavelength in a specific time slot is converted to an output optical signal having a different wavelength in a different time slot. It is preferable to exchange signals without converting to electric signals in the switching system.
A first conventional optical switching system has been disclosed by Suzuki et al. on pages 91 to 96 of the text SE87-146 of Switching Engineering Conference of Electronics and Information Communication, entitled "A Study on Wavelength-Division and Time-Division Composite Optical Communication Networks", 1987. In the first conventional optical switching system, optical wavelength-division and time-division multiplexed signals are exchanged by converting wavelengths and time slots of the input optical signals.
The first conventional optical switching system includes an optical splitter which divides an input optical signal to a plurality of optical signals, a plurality of tunable wavelength filters each of which selects one optical signal having a desired wavelength and a time slot, a plurality of optical bistable elements each of which converts a wavelength of each corresponding optical signal to a predetermined wavelength, and an optical switch which selectively supplies a selected signal from the converted signals supplied from the corresponding optical bistable element. The optical bistable element can hold the output intensity P.sub.out for a desired period by controlling conditions of the input intensity P.sub.in and the injected current i. Such an optical bistable element has been disclosed on pages 1088 and 1089 of Electronics Letters, No. 20, Vol. 23, Sep. 24, 1987. Additionally, the optical switch includes a plurality of optical gate switches.
In operation, the optical splitter supplies the tunable wavelength filters with divided input signals which are optical wavelength-division and time-division multiplexed signals. One of the tunable wavelength filters selects a signal having a desired wavelength from the optical wavelength-division and time-division multiplexed signals. The optical bistable elements are supplied with the current i which decreases to be reset periodically. If a signal having a specific wavelength allocated in a specific time slot is supplied to one of the optical bistable elements, the optical bistable element keeps supplying the optical switch with an output signal corresponding to the signal supplied thereto but converted to have a different predetermined wavelength for a period until the current i decreases. The optical switch supplies an optical output port with the output signal supplied from the optical bistable element in a desired time slot. Therefore, the input signal is exchanged to be an output signal having a different wavelength in a different time slot.
A second conventional optical switching system has been disclosed by Suzuki et al. in the text No. 29-2 of IEEE Global Communications & Exhibition, entitled "Photonic Wavelength-Division and Time-Division Hybrid Switching Networks for Large Line-Capacity Broadband Switching systems", 1988.
The second conventional optical switching system includes a wavelength filter which selects one optical signal having a predetermined wavelength from an input signal which is an optical wavelength-division and time-division multiplexed signal, an optical switch which selectively supplies a plurality of signals each of which is allocated in a different time slot, a plurality of tunable wavelength output optical bistable elements each of which converts a wavelength of each corresponding optical signal to a desired wavelength and keeps supplying the converted signal for a period, and an optical switch, each of which selectively supplies a plurality of signals each of which is allocated in a different time slot in turn. Structures of the optical bistable elements and the optical switch in the second conventional optical switching system are the same as those of the optical bistable elements and the optical switch in the first conventional optical switching system.
In operation, the wavelength filter selects a signal having a predetermined wavelength from the optical wavelength-division and time-division multiplexed signals. The tunable wavelength output optical bistable elements are supplied with the current i which decreases periodically to be reset. If a signal having a specific wavelength allocated in a specific time slot is supplied to one of the optical bistable elements, the optical bistable element keeps supplying the optical switch with an output signal corresponding to the signal supplied thereto but converted to have a different desired wavelength for a period until the current i decreases. The optical switch supplies the output signal supplied from the optical bistable element in a desired time slot. Therefore, the input signal is exchanged to be an output signal having a different wavelength in a different time slot.
According to the conventional optical switching systems for optical wavelength-division and time-division multiplexed signals, however, there is a disadvantage in that many active elements such as optical bistable elements are required to compose the system as wavelength-division multiplexity and time-division multiplexity numbers become larger. The reason will be explained.
Each of the optical bistable elements keeps supplying an output signal until the current i decreases, so that the optical switch for reading out optical signals is required to include optical gate switches corresponding to each optical bistable element in order to supply the output signals in turn to an optical combiner. Generally, if wavelength-division multiplexity and time-division multiplexity numbers are m and n, respectively, required number of the tunable wavelength filters, the optical bistable elements and the optical gate switches is m.times.n, respectively, so that required number of active elements is 3.times.m.times.n.
Further, there is another disadvantage in that the line-capacity is relatively small, because the conventional optical switching system includes only one input port and one output port, so that the line-capacity is limited by the wavelength-division multiplexity and time-division multiplexity numbers.